r - How to overlay geom_bar and geom_line plots with different number of elements using ggplot2? -

assuming have 2 data.frames different data in same range of x-values

a <-data.frame(x=c(1,1,1,2,2,2,3,3,3),                y=c(0.3,0.4,0.3,0.2,0.5,0.3,0.4,0.4,0.2),                 z=c("do","re","mi","do","re","mi","do","re","mi"))  b <- data.frame(x=c(1,2,3),y=c(10,15,8)) 

both, , b have same range of x values (1,2,3) while data.frame 9 rows, b data.frame 3 rows.

i use geom_bar in order plot distribution of values of a, this:

ggplot(a, aes(x=x, y=y, fill=z)) +     geom_bar(position="stack",stat="identity") +      ylab("") +      xlab("x") 

and use geom_line plot b data, this:

ggplot(b, aes(x=x, y=y)) +      geom_line(stat="identity") +      ylab("") + xlab("x") + ylim(0,15) 

now overlay geom_line plot previous geom_bar plot. first try following:

ggplot(a, aes(x=x, y=y, fill=z)) +     geom_bar(position="stack",stat="identity") +      ylab("") + xlab("x") +     ggplot(b, aes(x=x, y=y)) +      geom_line(stat="identity") +      ylab("") + xlab("x") + ylim(0,15) 

with no success.

how can overlay geom_line plot geom_bar plot?

try this

p <- ggplot()  p <- p + geom_bar(data = a, aes(x=x, y=y, fill=z), position="stack",stat="identity") p <- p + geom_line(data = b, aes(x=x, y=y/max(y)), stat="identity")  p 

update: can rescale 1 y make them same. don't know relations between 2 ys, rescaled them using y/max(y). solve problem?